1. Technical Field
Embodiments of the present invention relate generally to medical methods and, more particularly, but not exclusively, to a method for measuring heart rate variability (HRV).
2. Description of Related Art
Heart rate variability (HRV) is a well-known characteristic of heart function, and is a measure of variation in heart rate experienced by a particular patient.
HRV is usually measured as the time (in milliseconds) between successive heartbeats. This time period is also known as the “beat-to-beat interval”, and is the inverse of the instantaneous heart rate expressed in beats per second. FIG. 1 shows a standard ECG display of a series of heartbeats. It can be seen that each heartbeat appears as a wave, and that each wave in the series has points in common designated “Q”, “R”, and “S”. The point “R” is the highest peak and most visually distinguishable part of the wave, and therefore the easiest point from which to measure HRV. Accordingly, the beat-to-beat intervals may be designated as “RR intervals” or “peak intervals”, where “RR” is the interval between successive R's.
A patient heartbeat record will typically have thousands of heartbeats or waves. In order to extract HRV from this data, the waveforms are digitized and the RR intervals measured. HRV can then be conveniently represented in visual form by plotting the RR interval data as a non-linear recurrence or Poincaré plot, in which each data point is formed from a pair of successive RR intervals. HRV can be further quantified by applying a mathematical algorithm to the data points. Some of the algorithms that have been suggested however are complicated and/or fail to produce HRV values that are reliable in clinical applications.
The heart is connected to the autonomic nervous system, and the length of an RR interval is primarily governed by inputs from this system's two main components: the sympathetic nervous system and the parasympathetic nervous system. These latter two inputs to the heart are timed independently of one another in the normal course, and are also influenced by unrelated external factors such as the baroreceptor reflex, thermo-regulation, hormones, the sleep-wake cycle, eating, and stress. Accordingly, a certain degree of heart rate variability is to be expected in a healthy individual, and is a reflection of a well functioning autonomic nervous system. Similarly, a heart rate variability that is lower or otherwise different from the norm is often an indication of a malfunctioning autonomic nervous system.
As a result of this connection, HRV is often used in clinical settings to estimate the efferent autonomic activity to the heart and the integrity of this part of the cardiovascular control system. Problems with the autonomic nervous system are also associated with a variety of other medical conditions, such as, for example, congestive heart failure, Parkinson's disease, diabetic neuropathy, and Alzheimers. Accordingly, an HRV reading can suggest a risk of the patient acquiring such a condition in the future, or alternatively help evaluate the severity and progress of the condition where the patient has already been diagnosed.
HRV is also used to help evaluate and determine a course of treatment for patients who have had a serious heart attack, such as acute myocardial infarction (AMI). Post heart attack patients are at risk of ventricular tachycardia (VT), a fast heart rhythm that originates in a heart ventricle. VT is a potentially life-threatening arrhyhmia that may lead to ventricular fibrillation (VF) and sudden death. This combination of a VT episode followed by VF is sometimes referred to as “VT/VF”.
Patients considered to be at risk of VT/VF are often treated by receiving a surgically implanted device called an implantable cardioverter defibrillator (ICD). An ICD is a small, battery powered electrical impulse generator that continuously monitors a patient's heart rate and delivers a small electric jolt or shock when it detects a cardiac arrhythmia such as VT or VF. In some cases, instead of applying a shock, an ICD that detects VT will pace the heart at a faster rate, to break up the VT before it progresses to VF. A problem with ICD's however is that they sometimes respond to the arrhythmia too late, causing neurological damage or death. Another problem is that the ICD electric shocks, even if appropriately delivered, are painful and often unexpected by the patient. Yet another concern is simply the risk that is always present for a procedure that involves surgery.
An alternative therapeutic approach to VT/VF risk is to suppress an episode from occurring by putting the patient on long term anti-arrhythmic drugs such as beta-blockers or amiodarone. These drugs however are costly and have potent side effects.
The drawbacks of available therapies suggest the importance of obtaining an accurate diagnosis of a patient's post AMI risk for VT/VF. A patient that is wrongly diagnosed as low risk and that accordingly doesn't receive an ICD or drug treatment has a high likelihood of experiencing a fatal heart attack. Conversely, a patient that is wrongly diagnosed as high risk will experience some level of pain, trauma, cost, and/or surgical risk if implanted with an ICD or prescribed a drug unnecessarily. Physicians who evaluate a patient's post AMI risk for VT/VF sometimes consider the patient's HRV value, in combination with other factors, in making their decision. However, there continues to be an excessive variation among different practitioners in the effective rate of ICD implantation and appropriate drug therapy, in part due to inadequate risk stratification.